Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session H9: Biofluids: Microswimmers II - Boundary Effects |
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Chair: James Bird, Boston University Room: 3014/3016 |
Monday, November 24, 2014 10:30AM - 10:43AM |
H9.00001: Surfing wavy surfaces: Bacteria-surface interactions in flow Gast\'on L. Mi\~no, Vasily Kantsler, Roman Stocker Complex processes occur when microbes interact with surfaces, from mixture enhancement and motion rectification to biofilm formation. Microbe-surface interactions frequently occur in flowing fluids, and flow has recently been shown to have itself unexpected consequences on the dynamics of motile microbes. Here we report on microfluidic experiments in which the interactions of \emph{Escherichia coli} bacteria with wavy surfaces was quantified in the presence of fluid flow, a model system for naturally occurring topography of many real surfaces. We quantify surface interactions in terms of incident and scattering angles over a range of flow conditions, and compare results to the observations for a microchannel with straight walls. [Preview Abstract] |
Monday, November 24, 2014 10:43AM - 10:56AM |
H9.00002: Bacterial encountering with oil droplet Jian Sheng, Mehdi Molaei Encountering of microorganisms with rising oil droplets in aqueous environments is the first and one of the critical steps in the biodegradation of crude oil. Several factors such as droplet sizes, rising velocity, surfactant, and motility of bacteria are expected to affect the encounter rate. We establish well controlled microfluidic devices by applying layer-by-layer technique that allows us to produce horizontal micro droplets with different sizes. The encounter rates of passive particles, motile and non-motile bacteria with these droplets are measured by high speed microscopy. The effects of mobility and motility of these particles on encounter rates are assessed quantitatively. Meanwhile, we visualize reorientation of the particle due to flow filed around the oil droplet. Results show that the motile bacteria have higher probabilities to interact with an oil droplet compare to the passive particles. Ongoing analyses focus on the effect of shear rates, angular dispersion, curvatures of streamlines, and the swimming velocity of bacteria. The ratios of the encounter area to the entire droplet surface at various flow regimes will also been measured. [Preview Abstract] |
Monday, November 24, 2014 10:56AM - 11:09AM |
H9.00003: Active oil-water interfaces: buckling and deformation of oil drops by bacteria Gabriel Juarez, Roman Stocker Bacteria are unicellular organisms that seek nutrients and energy for growth, division, and self-propulsion. Bacteria are also natural colloidal particles that attach and self-assemble at liquid-liquid interfaces. Here, we present experimental results on active oil-water interfaces that spontaneously form when bacteria accumulate or grow on the interface. Using phase-contrast and fluorescence microscopy, we simultaneously observed the dynamics of adsorbed Alcanivorax bacteria and the oil-water interface within microfluidic devices. We find that, by growing and dividing, adsorbed bacteria form a jammed monolayer of cells that encapsulates the entire oil drop. As bacteria continue to grow at the interface, the drop buckles and the interface undergoes strong deformations. The bacteria act to stabilize non-equilibrium shapes of the oil-phase such wrinkling and tubulation. In addition to presenting a natural example of a living interface, these findings shape our understanding of microbial degradation of oil and may have important repercussions on engineering interventions for oil bioremediation. [Preview Abstract] |
Monday, November 24, 2014 11:09AM - 11:22AM |
H9.00004: Direct measurement of cell concentrations in bubble films prior to rupture Peter Walls, James Bird Pathogens or other solid particulates suspended in a liquid can attach to the interface of a bubble as it rises to the surface. When the bubble eventually ruptures at the free surface, these particulates can be ejected into film or jet droplets, such as those linked to the respiratory irritation experienced by shoreline residents during red tide events. Previous studies have demonstrated that the particulate concentration in these aerosols can be significantly higher than in the original liquid. However, the evolution from enriched film to enriched droplets is not entirely understood. Here we develop a physical model for the concentration enrichment by considering the concentration of particulates in the bubble film. In addition, we experimentally measure particulate concentration in the bubble film prior to rupture. The observed concentrations are consistent with the developed model. [Preview Abstract] |
Monday, November 24, 2014 11:22AM - 11:35AM |
H9.00005: Microorganism Billiards Colin Wahl, Joseph Lukasik, Saverio Spagnolie, Jean-Luc Thiffeault The presence of boundaries can have many different consequences on the locomotion of microorganisms. Recent experiments and numerical simulations have shown that certain types of microorganisms have a particular interaction with a wall: either through active (flagellar contact with the surface) or passive (hydrodynamic) interactions, the body rotates away from the surface and then departs at a critical angle. We explore the billiard-like motion of such a body as it swims in confined domains. The dynamics of swimming inside a regular polygon is characterized, where stable periodic or unstable chaotic trajectories are determined by the angle of departure. We also explore the dynamics of swimming in an array of obstacles. The results may provide insight on entrapment and sorting of microorganisms and other active particles. [Preview Abstract] |
Monday, November 24, 2014 11:35AM - 11:48AM |
H9.00006: Attraction of undulatory swimmers, such as nematodes, to surfaces Jinzhou Yuan, David Raizen, Haim Bau Nematodes play a significant role in the ecosystem; agriculture; human, animal, and plant disease; and medical research. The interactions between nematodes and surfaces may play an important role in nematodes' life cycle and ability to invade a host. We studied the effect of a surface on the dynamics of low-Reynolds number, undulating swimmers such as \textit{Caenorhabditis (C.) elegans -- }both wild type and touch-insensitive. The experiments demonstrated that swimmers located far from a surface selected randomly their direction of motion. In contrast, surface-proximate swimmers rotated towards, collided with, and swam along the surface for considerable time intervals, periodically contacting the surface with their anterior. Likewise, swimmers in a swarm were present at higher concentrations close to the surface. Both resistive force theory-based calculations and symmetry arguments predict that short range hydrodynamic torque, resulting from the interaction between the swimmer-induced flow field and the surface, rotate the swimmer towards the surface. We conclude that the surface attraction and following results from the interplay between short-range hydrodynamic and steric forces and is genotype-independent. [Preview Abstract] |
Monday, November 24, 2014 11:48AM - 12:01PM |
H9.00007: Motion of motile bacteria near a solid surface under shear flows Mehdi Molaei, Jian Sheng Shear is known to affect microorganism locomotion in several ways that includes rheotaxis, upstream motility, and periodic motion namely Jeffery orbits, which are crucial biological processes in biofilm formation. We investigate the effect of shear flow on the motility of \textit{E.coli} by employing microfluidic devices, high speed microscopy, and digital holography microscopy. Digital holography enables us to track the bacteria and obtain 3D swimming trajectories; meanwhile, high speed microscopy at different distances from the surfaces of the microfluidics allows us to visualize fast occurring phenomena such as cell reorientation by shear or tumbling events and subsequently to quantify the angular dispersion of active particle suspension. The result shows that Jeffery orbital motion for motile \textit{E. coli} is diametric different than that for passive bacteria. The results show that shear promotes bacterial re-orientation/tumbling near a solid surface whereas the tumbling is suppressed near a solid surface under quiescent flow condition. Ongoing analyses focus on determining whether this enhancement is the results of Jeffery orbital motion by the flow shear or the hydrodynamic interactions of bacteria with a solid surface. [Preview Abstract] |
Monday, November 24, 2014 12:01PM - 12:14PM |
H9.00008: Hydrodynamic entrapment, scattering, and escape of swimming bodies near colloidal particles Saverio Spagnolie, Gregorio Moreno Flores, Denis Bartolo, Eric Lauga Microorganisms and other self-propelling bodies in viscous fluids are known to traverse complex trajectories in the presence of boundaries, due to passive hydrodynamic and other physical effects. Motivated by the experimental findings of Takagi et al. on self-propulsion in a field of colloidal particles, we derive the far-field hydrodynamic interaction between model ``pusher'' and ``puller'' dipole swimmers and no-slip spherical bodies of varying size. Using the analytical estimates for the swimming trajectories, we predict the critical colloid size or dipole strength for which hydrodynamic entrapment occurs, the scattering dynamics for near-obstacle interactions, and the consequences of Brownian fluctuations. The dynamics include billiard-like motion between colloids, intermittent periods of entrapped/orbiting states near single colloids, and apparently randomized escape behavior. We envision applications of the theory to techniques for sorting microorganisms or other self-propelled swimmers, and to the behavior of motile suspensions in inhomogeneous environments. [Preview Abstract] |
Monday, November 24, 2014 12:14PM - 12:27PM |
H9.00009: Swimming near an interface in a viscoelastic fluid Shahrzad Yazdi, Arezoo Ardekani, Ali Borhan Given the versatility of their natural habitats, microorganisms often encounter the presence of confining boundaries while moving in polymeric solutions. Some examples include swimming of spermatozoa in the mammalian reproductive tract or bacteria in extracellular polymeric matrices during biofilm formation. It has been shown that both confinement and fluid elasticity can have significant impacts on the locomotion of microswimmers. However, the combined effect of these environmental conditions has not been fully understood yet. In this work, we present a fully resolved solution of a low-Reynolds-number microorganism swimming near an interface in a viscoelastic fluid. The kinematics of locomotion for a squirmer in a viscoelastic fluid is compared to its Newtonian counterpart using a perturbation analysis. The results suggest that extracellular polymers dramatically alter the swimming hydrodynamics, and in general increase the residence time of the microswimmer near a no-slip boundary that can consequently facilitate its adhesion rate. The emergence of a limit cycle can also enhance cell-cell communication in the form of quorum sensing and consequently biofilm formation. [Preview Abstract] |
Monday, November 24, 2014 12:27PM - 12:40PM |
H9.00010: Effect of solid boundaries on a motile microorganism in a viscoelastic fluid Alireza Karimi, Gaojin Li, Arezoo Ardekani Microorganisms swimming in viscoelastic fluids are ubiquitous in nature; this includes biofilms grown on surfaces, Helicobacter pylori colonizing in the mucus layer covering the stomach and spermatozoa swimming through cervical mucus inside the mammalian female reproductive tract. Previous studies have focused on the locomotion of microorganisms in an unbounded viscoelastic fluid. However in many situations, microorganisms interact with solid boundaries and their hydrodynamic interaction is poorly understood. In this work, we numerically study the effect of solid boundaries on the swimming behavior of an archetypal low-Reynolds number swimmer, called ``squirmer,'' in a viscoelastic fluid. A Giesekus constitutive equation is used to model both viscoelasticity and shear-thinning behavior of the background fluid. We found that the time a neutral squirmer spends in the close proximity of the wall increases with polymer relaxation time and reaches a maximum at Weissenberg number of unity. A pusher is found to be trapped near the wall in a viscoelastic fluid, but the puller is less affected. [Preview Abstract] |
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